Messages in this thread | | | Date | Thu, 30 Apr 2015 18:23:28 -0400 | From | Boris Ostrovsky <> | Subject | Re: [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests |
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On 04/30/2015 03:35 PM, Andy Lutomirski wrote: > On Thu, Apr 30, 2015 at 12:30 PM, Boris Ostrovsky > <boris.ostrovsky@oracle.com> wrote: >> On 04/30/2015 03:17 PM, Andy Lutomirski wrote: >>> On Thu, Apr 30, 2015 at 12:08 PM, Boris Ostrovsky >>> <boris.ostrovsky@oracle.com> wrote: >>>> Commit 61f01dd941ba ("x86_64, asm: Work around AMD SYSRET SS descriptor >>>> attribute issue") makes AMD processors set SS to __KERNEL_DS in >>>> __switch_to() to deal with cases when SS is NULL. >>>> >>>> This breaks Xen PV guests who do not want to load SS with__KERNEL_DS. >>>> >>>> Since the problem that the commit is trying to address would have to be >>>> fixed in the hypervisor (if it in fact exists under Xen) there is no >>>> reason to set X86_BUG_SYSRET_SS_ATTRS flag for PV VPCUs here. >>>> >>> Seems reasonable. >>> >>> Have you run the test case on a Xen PV guest on AMD? It's possible >>> that Xen is affected, since the old accidental workaround that we >>> deleted was in the vdso and probably would have worked on Xen. >> >> Is there a specific test that would trigger this bug? I >> booted/suspended/resumed a bunch of various guest types on both AMD and >> Intel but not much more than that. What is the commit that you deleted? >> >> I doubt that a change in the guest could suddenly start triggering this >> issue but I will take a look at hypervisor code. > http://article.gmane.org/gmane.linux.kernel/1937899 > > The change was that we stopped reloading ss after 32-bit sysret in the > vdso trampoline. Before we always reloaded ss in userspace before > doing any stack operations. > > I have no idea whether this was deliberate. It had done that (with > the attendant performance hit) since the very beginning, and there was > no comment explaining why.
The test passed. Which I guess is not surprising given that interrupt handling is done by the hypervisor.
-boris
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